Optical spectroscopic observations of intermediate-mass black holes and their host galaxies: the $M_{BH}-\sigma_*$ relation

TL;DR

This study investigates the $M_{BH}-\sigma_*$ relation for intermediate-mass black holes (IMBHs) using optical spectroscopy, revealing deviations from the established relation for larger SMBHs and suggesting different growth mechanisms.

Contribution

The paper provides new spectroscopic data for IMBH host galaxies and analyzes their position on the $M_{BH}-\sigma_*$ relation, highlighting differences from larger SMBHs.

Findings

IMBH host galaxies have $\sigma_*$ between 24 and 118 km/s.

IMBHs do not follow the $M_{BH}-\sigma_*$ correlation seen in larger SMBHs.

Accretion is likely the dominant growth channel for IMBHs.

Abstract

Intermediate-mass black holes (IMBHs; $M_{BH} <2*10^{5} M_{\odot}$) in galaxy centers are cruciel for painting a coherent picture of the formation and growth of supermassive black holes (SMBHs). Using Big Data analysis, we identified 305 IMBH candidates for IMBH and 1623 candidates of `light-weight' SMBHs ($2 * 10^{5} M_{odot} < M_{BH} <10^{6} M_{\odot}$). For 35 host galaxies from this combined sample with the X-ray-confirmed active galactic nuclei (AGN) we collected and analyzed optical spectroscopic observations. These data show that bulge stellar velocity dispersions ($\sigma_*$) lie in the range of 24$\dots$118~km/s and do not follow the correlation with $M_{BH}$ established by larger SMBHs indicating that in the $10^{5}-10^{6} M_{\odot}$ range the accretion is the prevailing BH growth channel.